Device for applying light to an inner surface of a cylinder and beam transformation device for such a device

ABSTRACT

A device ( 1 ) for applying light ( 4 ) to an inner surface ( 2 ) of a cylinder ( 3 ), comprising a homogenizer ( 14 ), into which light ( 4 ) can enter and from which the light ( 4 ) can exit, wherein the homogenizer ( 14 ) has a cylindrical internal surface ( 15 ), on which the light ( 4 ) can be reflected after entering and before exiting, and also comprising ways for introducing light ( 4 ) into the homogenizing means ( 14 ), and focusing arrangements, which can focus light ( 4 ) exiting from the homogenizer ( 14 ) onto the inner surface ( 2 ) of the cylinder ( 3 ) to which light ( 4 ) is to be applied.

The present invention relates to a device for applying light to an innersurface of a cylinder and to a beam transformation device that isparticularly suitable for such a device.

Definitions: The terms light or illumination or laser radiation are notintended to be limited to the visible spectral range of the light.Rather, the terms light or illumination or laser radiation in thecontext of this application are used for electromagnetic radiation overthe entire wavelength range from FIR to the XUV. Propagation directionof the laser beam or the light refers to an average propagationdirection of the laser radiation or light, in particular when this isnot a plane wave or is at least partially divergent. Laser beam, lightbeam, sub-beam or beam may refer not only to an idealized beam ofgeometrical optics, but also to a real light beam, such as a laser beamhaving a Gaussian profile or a modified Gaussian profile, such as asuper-Gaussian profile or a top hat profile, which does not have aninfinitesimally small beam cross-section, but rather an extended beamcross-section.

For example, the aforementioned device can be used to selectively applya circular intensity distribution of laser light to the inside of atube, in order to process this inside. For example, the inside of a tubecan be successively annealed with an intensity distribution traveling inthe longitudinal direction of the tube. In this way, for example, acoating on the inside of the tube can be treated.

It is the object of the present invention to provide a device of theaforementioned type which is capable of producing on the inside of thecylinder onto with light is to be applied a circular focus area with thegreatest possible homogeneity, wherein the focus range should inparticular be as thin as possible. Furthermore, a beam transformationdevice is to be provided which is suitable for such a device.

This is achieved with a beam transformation device according to claim 1or claim 2 and with a device for applying light to an inside of acylinder according to claim 4. The dependent claims relate to preferredembodiments according to the invention.

According to claim 1, it is provided that the beam transformation deviceincludes a plurality of cylindrical lens arrays which are arrangedannularly adjacent to one another and which each have cylindrical lenseswhose cylinder axes are aligned at an angle of 45° relative to theradial direction of the annular arrangement. Such a beam transformationdevice can rotate the unit vectors of the sub-beams of the light passingthrough the beam transformation device by 90° and/or cause a rotation ofthe azimuth angle of a light beam passing through the beamtransformation device by 90°.

According to claim 2, it is provided that the beam transformation deviceincludes at least one first cylindrical lens array having a plurality ofcylindrical lenses whose cylinder axes extend in a first direction,wherein light to be transformed can pass through the first cylindricallens array, and at least one second cylindrical lens array having aplurality of cylindrical lenses, whose cylinder axes extend in a seconddirection, wherein light exiting the first cylindrical lens array canpass through the second cylindrical lens array, and wherein the firstdirection encloses an angle of 45° with the second direction. Such abeam transformation device can rotate the unit vectors of the sub-beamsof light passing through the beam transformation device light by 90° orcause the azimuth angle of a light beam passing through the beamtransformation device the light beam to be rotated by 90°.

According to claim 4, it is provided that the device includeshomogenizing means, into which light can enter and from which light canexit again, wherein the homogenizing means has a cylindrical innersurface at which the light can be reflected after entering and prior toexiting, means for introducing light into the homogenizing means, andfocusing means configured to focus light exiting the homogenizing meanson the inside of the cylinder to which the light is be applied. Thehomogenizing means can attain a homogeneous intensity distribution ofthe light on the inside of the cylinder.

It may be provided that the homogenizing means are formed as a hollowcylinder, whereby in particular the inner jacket surface of the hollowcylinder is designed to be reflective. With this configuration, lightpropagating through the hollow cylinder can be homogenized due tomultiple reflections.

It may be provided that the inner jacket surface of the hollow cylinderis patterned. This patterning ensures good homogenization of thecircular focus area on the inside of the cylinder. It may be providedthat the patterning is arranged on the inner jacket surface as areflective array of hollow cylinder sections. In cross-section, areflective array of circular arcs is curled into a circle. Such apattern is thus similar to a flat array of cylindrical lenses commonlyused for homogenizing light.

It is possible to use, instead of a hollow cylinder, an in particularsolid light guide as homogenizing means. In this solid light guide, thelight can also be homogenized by multiple internal reflections.

It may be provided that the focusing means are formed as lens means, inparticular as a toroidal lens means, or as mirror means, in particularas a toroidal mirror means, or include a lens means, in particular atoroidal lens means, and/or a mirror means, in particular a toroidalmirror means. Such focusing means can ensure a circular focus area onthe inside of the cylinder with a comparatively small line width.

It is possible that the device comprises a beam transformation devicewhich is arranged upstream of the homogenizing means so that the lightcan enter the homogenizing means after passing through the beamtransformation device. In particular, the beam transformation device canbe a beam transformation device according to claim 1 or claim 2. Therotation of the unit vectors or of the azimuth angle of the light by 90°caused by such a beam transformation device, adds a sagittal componentto the light that enhances the homogenization effect of the homogenizingmeans.

It may be provided that the device includes beam expansion means thatexpand and/or shape the light prior to entering the beam transformationdevice and/or the homogenizing means such that the beam cross-section oflight has off-center an intensity maximum or several intensity maxima.Such a measure can simplify focusing to a circular focus area on theinside of the cylinder.

Further features and advantages of the present invention will becomeapparent from the following description of preferred exemplaryembodiments with reference to the accompanying drawings, which show in:

FIG. 1 a schematic side view of a first embodiment of a device accordingto the invention;

FIG. 2 a schematic side view of a second embodiment of a deviceaccording to the invention;

FIG. 3 a distribution of beam trajectories typical for meridional raysin a projection onto a plane perpendicular to the cylinder axis of thehomogenizing means;

FIG. 4 a distribution of beam trajectories typical for sagittal rays ina projection onto a plane perpendicular to the cylinder axis of thehomogenizing means;

FIG. 5 a distribution of beam trajectories typical for the embodimentsshown in FIG. 1 and FIG. 2 in a projection onto a plane perpendicular tothe cylinder axis of the homogenizing means;

FIG. 6 a perspective view of a homogenizing means;

FIG. 7 a schematic view showing the unit vectors of light in aprojection onto a plane perpendicular to the mean direction of lightpropagation prior to entering in the beam transformation device;

FIG. 8 a schematic view showing the unit vectors of light in aprojection onto a plane perpendicular to the mean direction of lightpropagation after exiting from the beam transformation device;

FIG. 9 a schematic side view of a first embodiment of a beamtransformation device according to the invention;

FIG. 10 a perspective view of a second embodiment of a beamtransformation device according to the invention;

FIG. 11 a schematic side view of the second embodiment of a beamtransformation device according to the invention;

FIG. 12 an illustration of the effect of the second embodiment of a beamtransformation device according to the invention:

FIG. 13 a schematic side view of a third embodiment of a beamtransformation device according to the invention;

FIG. 14 a schematic side view of the optical paths of the light in theupper region of a device according to the invention using three lightsources;

FIG. 15 a schematic side view corresponding to FIG. 14 of the opticalpaths of the light in the lower region of the device;

FIG. 16 a schematic side view of the optical paths of the light in thelower region of the device corresponding to FIG. 14, rotated by 90° withrespect to FIG. 15;

FIG. 17 a schematic crass-section through the light in the region of thelight sources when using three light sources;

FIG. 18 a schematic cross-section through the light after the collimatormeans;

FIG. 19 a schematic cross-section through the light after thecompressing means;

FIG. 20 a schematic cross-section through the light upstream of he beamtransforming means;

FIG. 21 an exemplary local intensity distribution of the light on theinside of the cylinder to which light is to be applied, in the eventthat no beam transformation device is used;

FIG. 22 an exemplary local intensity distribution of the light on theinside of the cylinder to which light is to be applied, in the eventthat a beam transformation device according to the invention is used;

FIG. 23 an overall intensity distribution of the light on the inside ofthe cylinder to which light is to be applied, in the event that nohomogenizing means are used;

FIG. 24 an overall intensity distribution of the light on the inside ofthe cylinder to which light is to be applied, in the event thathomogenizing means are used.

In the figures, identical or functionally identical parts or light beamsare provided with the same reference symbols.

The embodiment of a device 1 according to the invention depicted in FIG.1 is used for applying light 4 to an inner surface 2 of a schematicallyillustrated cylinder 3. In particular, a circular focus area 5 is to beformed with the device 1 according to the invention on the inner surface2 of the cylinder 3. The device 1 is in the illustrated embodimentlocated inside the cylinder 3.

In the illustrated embodiment, the device 1 includes four light sources6 which may, for example, be the ends of optical fibers, wherein laserlight can be coupled into the optical fibers. The light 4 emanating fromthe light sources 6 (see exemplary distribution with three light sourcesin FIG. 17) is collimated by collimating means 7 (see exemplarydistribution with three light sources in FIG. 18) and reflected bymirrors 8 onto compressing means 9, wherein the compressing means 9 arerealized by a reflective four-sided pyramid. FIG. 19 shows an exemplarydistribution for three light sources downstream of the compressing means9.

The device further includes a cone 10 with a reflective outer surfaceonto which the compressed light 4 is reflected by the pyramid. Startingfrom this cone 10, the light 4 is directed radially outwards onto thereflective inner surface of a hollow cone 11 where the light isreflected upwardly in FIG. 1, so that the light 4 now propagates againin the axial direction of the cylinder 3.

The cone 10 with the reflective outer surface and the hollow cone 11with the reflective inner surface together form beam expansion meansconfigured to expand and shape the light 4 so that the beamcross-section of the light has off-center an intensity maximum orseveral intensity maxima. In this context, see the exemplarydistribution with three light sources in FIG. 20.

A first embodiment of a beam transformation device 12 according to theinvention is arranged downstream of the hollow cone 11 in thepropagation direction of the light 4. The beam transformation device 12is shown again in more detail in FIG. 9.

The beam transformation device 12 includes a plurality of cylindricallens arrays 121, each having a plurality of cylindrical lenses 122. Theindividual cylindrical lens arrays 121 are arranged in a ring. Each ofthe cylinder axes Z of the cylindrical lenses 122 is orientedapproximately at an angle γ of 45° relative to the radial direction R ofthe ring.

The individual cylindrical lenses 122 are formed, for example, asbiconvex lenses with a convex surface on the entrance side and a convexsurface on the exit side of the beam transformation device 12. Here, themutual distance of these two convex surfaces to each other correspondsin particular to the sum of the focal lengths of these two convexsurfaces or to twice the focal lengths of the convex surfaces if thefocal lengths are equal. Each of the cylindrical lenses 122 then forms aKepler telescope.

FIG. 7 and FIG. 8 each show respective schematic views illustrating theunit vectors of sub-beams of the light 4 in a projection onto a planeperpendicular to the mean direction of propagation of the light 4 beforeentering into and after exiting from the beam transformation means 12.As can be seen, the unit vectors are rotated by 90° by the cylindricallenses that are oriented at an angle of 45° relative to the radialdirection.

This adds a sagittal component to the previously collimated sub-beams ofthe light 4. FIG. 3 to FIG. 5 illustrate this relationship. FIG. 3 toFIG. 5 show distributions of beam trajectories in a projection onto aplane perpendicular to the cylindrical axis of a cylindrical medium inwhich the light propagates. FIG. 3 illustrates a distribution typicalfor meridional rays and FIG. 4 shows a distribution of beam trajectoriestypical for sagittal rays.

The sub-beams of the light 4 have for full collimation essentially onlya meridional component. When passing through the beam transformationdevice 12, a sagittal component is added to the sub-beams of the light4. Such distribution having a sagittal component is shown in FIG. 5.

Another hollow cone 13 with a reflective inner surface is arranged inthe propagation direction of light 4 downstream of the beamtransformation device 12. The light 4 is reflected at the inner surfaceof the hollow cone 13 toward a homogenizing means 14, as shownschematically in FIG. 6.

The homogenizing means 14 is constructed as a hollow cylinder with areflective, patterned inner surface 15. The patterns of the innersurface 15 are concave cylinder sections with cylinder axes extendingparallel to the cylinder axis of the hollow cylinder. The concavecylinder sections are arranged consecutively in the circumferentialdirection of the hollow cylinder. The homogenizing means 14 can besignificantly longer than shown in the schematic diagram of FIG. 6.

The light 4 is homogenized in the homogenizing means 14 by multiplereflections on the inner surface 15 so as to produce substantially thesame intensity along the entire circular focus area 5. Thehomogenization is enhanced by admixing to the light 4 with the beamtransformation device 12 a sagittal component, as described withreference to FIG. 3 to FIG. 5.

FIG. 21 illustrates how low the homogeneity is without the beamtransformation device 12. In contrast, FIG. 22 shows a very homogeneousdistribution of the light 4 across the circular focus area 5. FIG. 23shows how low the homogeneity is without the homogenizing means 14. Incontrast, FIG. 24 shows a very homogeneous distribution of the light 4across the circular focus area 5.

The patterning of the homogenizing means 14 also ensures goodhomogenization of the circular focus area 5 on the inner surface 2 ofthe cylinder 3.

The light 4 exiting from the homogenizing means 14 is focused by a lensmeans acting as a focusing means 16 onto the inner surface 2 of thecylinder 3 onto which light 4 is to be applied. The lens means 16 isformed in particular as a toroidal lens means 16 producing a circularfocus area 5 on the inner surface 2 of the cylinder 3. The toroidal lensmeans 16 includes a peripheral torus-shaped outer surface 17, by whichthe light 4 is refracted such that the light 4 is focused on the innersurface 2 of the cylinder 3.

The second embodiment shown in FIG. 2 differs from the first embodimentshown in FIG. 1 in that a mirror means 18 also operating as a focusingmeans is employed instead of the lens means 16. The mirror means 18 isconstructed in particular as a toroidal mirror means 18, so that thetoroidal mirror means 18 also produces a circular focus area 5 on theinner surface 2 of the cylinder 3. In this case, the toroidal mirrormeans 18 has a reflective toroidal inner surface 19, by which the light4 is reflected onto the inner surface 2 of the cylinder 3.

FIG. 10 and FIG. 11 show a second embodiment of a beam transformationdevice 20 according to the invention. This second embodiment includestwo consecutively arranged cylindrical lens arrays 21, 22, wherein thecylinder axes of the cylindrical lenses 23, 24 of these cylindrical lensarrays 21, 22 enclose an angle of 45° with one another.

Also in this embodiment, the individual cylindrical lenses 23, 24 areeach formed for example as biconvex lenses with a convex surface on theentrance side and a convex surface on the exit side of the beamtransformation device 20. The mutual distance between these two convexsurfaces to each other corresponds in particular to the sum of the focallengths of these two convex surfaces or to twice the focal length of theconvex surfaces if the focal lengths are equal. Each of the cylindricallenses 23, 24 forms hereby a Kepler telescope.

In FIG. 12, the azimuth angle of an incident light beam is denoted byA₁, and the azimuth angle of the exiting light beam by A₃. A₂ indicatesthe azimuth angle of the light beam after exiting from the firstcylindrical lens array 21. Furthermore, FIG. 12 shows the directions Z₂₃and Z₂₄ of the cylinder axes of the cylindrical lenses 23, 24 of thecylindrical lens arrays 21, 22. These enclose an angle of 45° with eachother.

FIG. 12 illustrates how the azimuth angle of a light beam passingthrough the beam transformation device 20 is rotated by the twocylindrical lens arrays 21, 22 together by 90°. In this case, the anglea between the azimuth angle A₁ and the direction Z₂₃ is transformed bythe first cylindrical lens array 21 by an angle −α. Thereafter, theangle β between the azimuth angle A₂ and the direction Z₂₄ istransformed by the second cylindrical lens array 22 by an angle −β.

Accordingly, due to the angle of 45° between the directions of Z₂₃ andZ₂₄ of the cylinder axes of the cylindrical lenses 23, 24, the azimuthangle of the light beam passing through the beam transformation device20 is rotated by 90°.

FIG. 13 shows another embodiment of a beam transformation device 25,which is constructed similar to the beam transformation device 20 shownin FIG. 10 and FIG. 11, but includes four instead of two cylindricallens arrays. In particular, two first cylindrical lens array 21 and twosecond cylindrical lens arrays 22, which are arranged consecutively inthe propagation direction of the light, are provided in the beamtransformation device 25.

1-11. (canceled)
 12. A beam transformation device (12), comprising aplurality of cylindrical lens arrays (121), arranged annularly adjacentto one another and each comprise respective cylindrical lenses (122)having cylinder axes (Z) oriented at an angle (γ) of 45° with respect tothe radial direction (R) of the annular arrangement.
 13. A device (1)for applying light (4) to an inner surface (2) of a cylinder (3),comprising homogenizer (14), into which the light (4) enters and fromwhich the light (4) exits again, wherein the homogenizer (14) has acylindrical inner surface (15), at which the light (4) reflects afterentering and before exiting, arrangement for introducing light (4) intothe homogenizer (14), focusing arrangement configured to focus light (4)exiting the homogenizer (14) onto the inner surface (2) of the cylinder(3) to which the light (4) is to be applied, a beam transformationdevice (12, 20, 25) arranged upstream of the homogenizer (14), so thatlight (4) that has passed through the beam transformation device (12,20, 25) enters into the homogenizer (14), wherein the beamtransformation device (12, 20, 25) is a beam trans for nation device(12, 20, 25) according to claim
 12. 14. The device according to claim13, wherein the homogenizer (14) are formed as a hollow cylinder,whereby in particular the inner jacket surface of the hollow cylinder isdesigned to be reflective.
 15. The device according to claim 14, whereinthe inner jacket surface of the hollow cylinder is patterned.
 16. Thedevice according to claim 15, wherein the patterning is arranged on theinner jacket surface as a reflective array of hollow cylinder section.17. The device according to claim 13, wherein the focusing arrangementare formed as lens arrangements (16).
 18. The device according to claim13, wherein the device comprises beam expanding arrangements, whichexpand and/or shape the light (4) prior to entering the beamtransformation device (12, 20, 25) and/or the homogenizer 14) so thatthe beam cross-section of the light (4) has off-center an intensitymaximum or several intensity maxima.
 19. The device according to claim17, wherein the ns arrangements (16) is formed as toroidal lensarrangements.
 17. The device according to claim 17, wherein the lensarrangements (16) is formed as mirror arrangement (18).
 18. The deviceaccording to claim 17, wherein the mirror arrangement (16) comprisestoroidal mirrors (18).
 19. The device according to claim 17, wherein themirror arrangement (16), comprising toroidal mirror arrangements (18).